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How Kīlauea’s lava birthed an algal bloom visible from space

Lava descending into Hawai‘i’s ocean drove an upward surge of deep sea nutrients, cultivating life at the surface.

ByKatherine J. WuNOVA NextNOVA Next
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Lava flowing into the ocean off the coast of Hawai‘i’s Big Island in the spring of 2018. Image Credit: Rachel Blaser, Shutterstock

Last spring, Hawai‘i’s Kīlauea volcano commenced a monthslong eruption, choking the air with ash and belching out rivers of fast-flowing lava. The cataclysmic event—wrought by the most hazardous volcano in the United States—prompted the evacuation of thousands of residents and destroyed more than 700 homes.

But even as Kīlauea wreaked havoc on land, it was quietly seeding new life at sea.

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Within weeks of the eruption, the ocean began to teem with a massive, blue-green whorl of phytoplankton—a type of algae that forms the foundation of many marine food chains—in a bloom so big it was visible from outer space. Now, a team of scientists has uncovered the unexpected trigger behind these microorganisms’ explosive growth: the same lava sowing chaos ashore. As the molten rock plunged into the sea, it warmed deep, nutrient-rich ocean waters, buoying them up to hungry cells at the surface.

The lava’s revitalizing churn, described in a paper published today in the journal Science, reveals a new way volcanic eruptions can reshape local ecosystems. It also hints at the myriad ways in which life can thrive, under even the strangest—and perhaps most devastating—of circumstances.

“Kīlauea has a destructive nature,” says study author Sam Wilson, a microbial oceanographer at the University of Hawai‘i at Manoa. “But it also gave us the opportunity to witness how land and ocean interact to create life.”

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An aerial photo of Hawai‘i’s coastline, taken on June 28, 2018. Lava entering the ocean is visible in the upper lefthand corner, creating water discoloration in the foreground. Image Credit: United States Coast Guard, Honolulu Office

In the weeks following the collapse of Kīlauea’s Pu‘u ‘O‘o crater, 200 billion gallons of lava—enough to fill 320,000 Olympic-sized swimming pools—gushed from the many fissures that opened on the volcano’s flanks.

Eventually, some of the lava reached the coastline, sending opaque plumes of laze—a noxious blend of steam, hydrochloric acid, and shards of volcanic glass—billowing over the spots where molten rock met ocean. But as the volcano’s innards continued to surge to the sea, a team of oceanographers had their sights set on something taking shape a bit further offshore: a greenish swirl of chlorophyll, blooming miles off the Big Island’s coast.

In the ocean, chlorophyll—the same green pigment found in plants—is the calling card of phytoplankton: tiny, light-dependent creatures found in the upper sun-soaked layers of Earth’s seas. Most phytoplankton are microscopic, but when they accumulate in high enough numbers, they can collectively warp the color of large swaths of seawater in a burst of growth called a bloom.

That’s not something that happens every day in this part of the Pacific, says study author Nick Hawco, a chemical oceanographer at the University of Southern California. Visible blooms of phytoplankton tend to occur in places where deep, nutrient-rich layers of the ocean do a lot of mixing with the resource-poor waters at the surface—a process called upwelling. In Hawai‘i’s neck of the woods, however, the surface of the sea is especially warm, creating a buoyant layer that thwarts the ascent of life-sustaining water from below.

But in the wake of a natural disaster, all bets are off—and it seemed something about the way Kīlauea had blown its top had prompted phytoplankton to forge their own eruption at sea.

So when a group of scientists led by David Karl, a microbial oceanographer at the University of Hawai‘i at Manoa, first noticed the blue-green bloom in a series of satellite images, they leapt at the chance to study the rare event in real time. A month after the waters first turned turquoise, the researchers began collecting samples of the bloom in hopes of pinpointing its cause.

A handful of previous studies had already hinted at a tantalizing culprit: nutrients like iron and phosphate from the lava itself, nourishing the phytoplankton like fertilizer. But as data from the team’s analysis started to trickle in, it quickly became clear that the original hypothesis wasn’t going to pan out.

While the seawater did contain some extra iron and phosphate, the big surprise was actually another life-sustaining compound: nitrate, present at concentrations hundreds of times higher than normal—enough to sustain the majority of the recent algal growth spurt. But unlike iron and phosphate, nitrate isn’t a major ingredient in lava. And the most abundant phytoplankton in the bloom, a group called Skeletonema, weren’t capable of synthesizing it themselves.

The question, then, became: Where was the nitrate coming from?

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A specimen of Skeletonema, a chain-forming phytoplankton. Skeletonema is especially common in marine environments that experience a lot of upwelling, a process in which deep, nutrient-rich waters mix with sunlit waters at the surface. Image Credit: Dr. Yaqin "Judy" Li, Milford Laboratory/NEFSC, NMFS/NOAA, flickr

By this point, the bloom had swollen into a whiplike tendril stretching some 100 miles offshore. That’s a lot of nitrate to account for, Wilson says. “Oceanographers know enough about the oceans to make predictions. But we don’t know enough that our predictions are always correct.”

Thus began months of detective work back in the lab. While nitrate is normally rare in sunlit parts of the ocean, groundwater can occasionally ferry the compound from septic tank to sea. But the chemical fingerprint of the bloom’s nitrate didn’t match the one normally found in sewage, Hawco says.

Another possibility involved lava converting pure nitrogen into nitrate at extremely high temperatures. But when Wilson sent a sample of Kīlauea’s molten rock into a kiln in the University of Hawai‘i’s art department, he didn’t get much out apart from very, very hot lava. And once again, the chemical signature was a miss.

What the nitrates’ chemical composition did resemble, however, was that of nitrates from deep ocean water, Hawco says—a final option that had, ironically, been under the team’s nose the entire time. “When we sat down and looked at the data, a lot of us were confused,” he says. “In this environment, we didn’t think deep waters ever reached the surface.”

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Lava entering the sea along the southern Kapoho coastline of Hawai‘i in 2018. Image Credit: United States Geological Survey, Wikimedia Commons

In the end, though, the pieces fit together, Wilson says. Between June and August, lava regularly cascaded off Hawai‘i’s coast at 25,000 gallons per second. At these speeds, the molten rock probably had no trouble reaching depths below 1,000 feet, where the sea is cool and full of nutrients. Warmed by the impact, deep waters would have risen to the surface, bringing compounds like nitrate along with them. It was, in a sense, old school upwelling—with an extremely explosive twist.

“I think they’ve pieced together a very convincing argument,” says Heather Bouman, a biogeochemist and phytoplankton researcher at the University of Oxford who wasn’t involved in the study. “This is a completely new mechanism by which volcanic eruptions can lead to ocean fertilization.”

Kīlauea’s eruptive fervor eventually subsided—and as the lava flows slowed to a trickle, the bloom ebbed and eventually disappeared. But the phytoplankton may still have left their mark on the local ecosystem, Hawco says. Given how critical these tiny creatures are to ocean biodiversity, their brief boom may have boosted other marine populations farther up the food chain.

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Lava entering the ocean off the southeast coast of Hawai‘i, generating plumes of laze—a noxious combination of steam, hydrochloric acid, and shards of volcanic glass. Image Credit: Karin Bjorkman

It’s also possible that the bloom offset an appreciable chunk of the Kīlauea’s recent carbon emissions, says Terry Plank, a volcanologist at Columbia University’s Lamont-Doherty Earth Observatory who co-authored a commentary on the study. Phytoplankton naturally take up carbon, dragging it down into the sea’s depths when they die. According to the calculations of Plank and marine biogeochemist Hugh Ducklow, also of Lamont-Doherty, these little organisms could play a bigger role in mediating the global warming effects of volcanic eruptions than once thought. (Notably, volcanoes don’t have a huge carbon footprint to begin with, and can also have their own cooling effects on the planet.)

So perhaps it’s good news that lava-driven upwelling might be pretty common. Some especially tantalizing candidates for further study, Wilson says, are the thousands of underwater volcanoes constantly stirring things up on the seafloor.

If this process is as widespread as the researchers think, the new findings may have the potential to improve prediction models spanning fields from climatology to oceanography, says Megumi Chikamoto, a paleoclimatologist at The University of Texas at Austin who was not involved in the study.

For now, though, “it’s so cool to see lava driving such a broad biological response,” Plank says. “Lava [may not seem] conducive to life...but it can cause the biosphere to thrive. Despite their bad rap, there’s good that comes from volcanic eruptions.”

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